Whether someone bitten by a sandfly goes on to develop the most lethal form of leishmaniasis is determined partly by the victim’s own genes, a new study suggests.

Leishmaniasis, caused by parasites injected by sandfly bites, has two forms: painful skin sores (known to American troops in Iraq as “Baghdad boils”) or, in less than 20 percent of cases, the visceral form, sometimes called “kala azar,” that attacks the organs and is fatal if untreated. About 400,000 visceral cases develop annually, 90 percent of them in three places far from one another and with different parasite subspecies: northeastern Brazil, the India-Bangladesh border and the Horn of Africa.

Because the disease hits some families harder, a genetic propensity to get it has been long suspected. The study, published in Nature Genetics last week, compared DNA in almost 6,000 blood samples from India and Brazil. Both Indians and Brazilians who got visceral leishmaniasis had similar DNA variations.

Researchers are not sure what those mutations do, but the nearest stretch of DNA determines how white blood cells “grab” invaders to trigger immune responses, said Peter J. Donnelly, an Oxford statistics professor who also heads the Wellcome Trust Center for Human Genetics and was one of the paper’s authors.

While it is widely known that immune-system genes influence susceptibility to cancer, learning that they may also control susceptibility to a parasitic disease was “quite interesting,” Dr. Donnelly said.

A version of this article appears in print on , Section D, Page 6 of the New York edition with the headline: Leishmaniasis: A Genetic Link Found in Far-Flung Victims Of a Lethal Form of a Parasitic Disease. Order Reprints | Today’s Paper | Subscribe